>I think some virus or Trojan got into my computer and now I only see one drive C: whereas before I had drivers D: and E:. The drive C: only shows very little space left. When I run the Windows Security Essentials it says that it cannot clean the computer and recommends downloading the Standard System Sweeper from some site. Does this sound as legitimate?

There is such an app from Microsoft Connect..although I've never tried it.

This of any help?
http://www.technobuzz.net/fix-missing-drive-in-windows-7/